Compression mechanism and pressure-induced amorphization of γ-ZrW2O8

The structure of gamma-ZrW2O8 has been optimized at zero pressure and also at V/V-0=0.97 by means of density functional theory calculations using the B3LYP functional. As previously found for alpha-ZrW2O8, tungsten polyhedra are stiffer than zirconium octahedra in gamma-ZrW2O8. However, contrary to what has been found for alpha-ZrW2O8, all first coordination polyhedra in the gamma phase are less compressible than the unit cell. Volume reduction in gamma-ZrW2O8 is, thus, mainly accomplished by polyhedral tilting. Upon pressure increase, the distance between the terminal oxygen and W atoms from the nearest polyhedra decreases by as much as 3.66% (for the pair O101-W6). Accordingly, a further reduction in the zirconium tungstate molar volume with the high-pressure transition to the amorphous phase should bring several oxygen atoms within the threshold of bond formation to W. O 1s photoelectron spectra provide further experimental evidence on the formation of additional W-O bonds in amorphous zirconium tungstate. These new W-O bonds should enable the metastable retention of the amorphous phase upon pressure release.